1. Field of the Invention
The present invention relates to a gas sensor, and more particularly, to a connection device for providing a heat-resistant joint between a ceramic element and lead strips.
2. Discussion of the Related Art
Oxygen sensor elements are known in the art to be made from ceramic elements. Commonly, ceramic elements are used in vehicles to determine the oxygen concentration of exhaust gases. Because the ceramic element is attached to an exhaust pipe, it will be exposed to high temperatures. FIG. 10 shows a configuration of the type disclosed in Japanese Laid-Open Patent Publication No. 10-253579 and Published European Patent Application No. EP 836094 (the disclosures of which are hereby incorporated by reference) having improved heat stability in an oxygen sensor element with a ceramic element.
As shown in FIG. 10, a ceramic element 30 to be attached to an exhaust pipe (not shown) is electrically connected to lead strips 38 by fitting a metallic ring 31 thereto with an insulation sheet 39 being disposed between the metallic ring 31 and the lead strips 38. Here, the metallic ring 31 is provided to have an appropriate interference amount and an appropriate minimum wall thickness.
Size reduction of the sensor is accompanied by reduction in the distance between an attachment portion of the sensor to an exhaust pipe and a connection portion between the ceramic element 30 and lead strip 38. Consequently, the connection portion is exposed to even higher temperatures.
The ceramic element 30 has been found to be strong with respect to compressive stresses, but weak with respect to tensile stresses. When the electrodes of lead strip 38 and the ceramic element 30 are joined through interference fitting, compressive stresses act on the joint portion of the ceramic element 30, which poses no problem. However, when the sensor is used at high temperatures, the joint portion undergoes thermal fatigue, thereby causing differences in thermal expansion among the elements of the sensor. The differences in thermal expansion result in the formation of cracks x, as shown in FIG. 10. In the worst case, a malfunction of the ceramic element 30 will result. When the interference amount is decreased in order to avoid the crack problem, a retaining force for the electrodes of the lead strips 38 decreases, thereby failing to reliably join the electrodes and the ceramic element 30.